This invention relates to a process for shaping thermoplastic articles formed in a rapid-stamping operation. More particularly, the invention relates to shaping thermoplastic compositions in which the transfer of the heated or molten polymer to a stamping press is expedited by means of a metal foil carrier or support.
It is known that many thermoplastics can be formed at ambient temperatures by means of various sheet metal-forming techniques such as deep drawing, stretch forming, stamping, etc. These relatively cold stamping processes are economically attractive because it is possible to achieve rapid production rates, e.g. rates that exceed one item every twenty seconds or even faster. In order to overcome the deficiencies of prior art compositions, e.g. dimensional instability, poor heat resistance, poor mechanical properties, reinforcement in the form of glass fibers are added to the thermoplastic composition. Such reinforced materials have higher mechanical strength, impact resistance and dimensional stability than unreinforced compositions.
The stamping of a glass mat reinforced thermoplastic sheet is described, for example, in U.S. Pat. Nos. 3,621,092 and 3,626,053. Novel variations of the basic stamping process are described, for example, in U.S. patent application Ser. Nos. 268,400, filed July 3, 1972 and 195,450, filed Jan. 11, 1971.
Basically, the rapid stamping process contemplated by the invention may be separated into the following stages:
1. Preheat of sheet or blank
2. Transfer to the rapid stamping press
3. Stamping under pressure
4. Removal or ejection from the mold
In accordance with the present invention, the first two stages are modified by the utilization of a foil carrier or support, which subsequently may perform an integral function in stage 3 and/or 4.
In Stage 1, the composite sheet, blank or charge is suitably heated such as in a radiant, dielectric, infrared, convection or vacuum oven or similar heating source, to a temperature above the melting point of the thermoplastic resin component of the blank.
In Stage 2, the hot charge or blank is transferred (by means of the foil support) to a mold placed in a stamping press, wherein the mold or set of dies impart the desired configuration of the final product to the sheet or blank. The mold is maintained at a temperature between about room temperature (23.degree. C.) and approximately 160.degree. C. or higher, depending on the polymer constituent of the sheet and upon the desired stamping characteristics.
In Stage 3, the press is rapidly closed for a period of time sufficient to cause the blank to conform to the shape of the mold and to cool and/or crystallize sufficiently to allow part removal without distortion.
In Stage 4, the molded article is removed from the mold, allowed to cool and transferred for further assembly or packaged for shipment.
It is readily recognized that stamping parameters such as pressure requirements, residence time in the mold, pre-heat temperature, etc. are dependent upon the sheet composition, thickness, part complexity, etc., as described to a large extent in U.S. patent application Ser. Nos. 268,400, filed July 3, 1972; 194,469, filed Nov. 1, 1971; and 195,450, filed Jan. 11, 1971.
A forming process of this kind has many advantages over prior art forming methods and compositions and includes, for example, the following advantages. The cycle times are extremely fast, with a cycle time of 10-30 seconds per part, even on extremely large parts (10-20 ft.sup.2 in area), being realizable.
Standard sheet-metal stamping, hydraulic or mechanical stamping presses are useable, although slight modifications may be required in the clutch assembly in order to obtain the desired pressure cycle.
The shaped articles may have complex and non-planar configuration.
Variable thickness relative to the initial charge thickness can be achieved during stamping.
Holes and notches can be formed during stamping or in post-forming operations.
The process, as previously practiced in the prior art, suffers from several deficiencies in the processing requirements. The transfer stage of the process (without the use of foil support) is a relatively clumsy and unmanageable step, often resulting in poorly formed parts. When the material is in sheet form, the sheet itself must be clamped, held, or otherwise supported during the transfer and preheat stages. Clamps or grid supports may cool, distort, puncture or otherwise damage the sheet during the pre-heat and transfer stages, resulting in an imperfect or poorly formed article. Special clamping devices, heating devices, non-stick fixtures and the like must be used to avoid large-scale damage to the pre-heated composition.
In addition, in order to keep the thermoplastic sheet intact during the physically rigorous pre-heat and transfer stages, it is necessary, in providing some integrity to the molten sheet, that some special means such as glass fiber reinforcement in the form of fibers of substantial length, usually greater than 11/2", and preferably continuous in length, be used. In the absence of the long fibers or glass cloth mat, or web, practical transfer of the molten thermoplastic compositions to the mold is difficult. Thus, since temperatures in excess of the melting point are often employed to soften the interior of the sheet, unless long glass fiber reinforcement is present to retain the sheet integrity and strength during the pre-heat stage, the sheet may virtually disintegrate during the pre-heat and/or transfer stages because of insufficient integral strength in the melt when heated above the melting point. Moreover, a fairly high amount of glass mat or similar reinforcement has heretofore frequently been required in the thermoplastic sheet in order to avoid dripping of the heated thermoplastic away from the sheet. Such a high concentration of glass mat inevitably leads to a poor surface finish on the final stamped part because of fiber prominence.
Still another shortcoming of long-glass fiber-filled compositions is that they often possess poorer mechanical properties when compared to short fiber-reinforced thermoplastics. Although the reinforcement efficiency theoretically increases as the reinforcing fiber length increases, poor wetting of the fiber by the thermoplastic matrix will negate this effect. If the fibers are present in the form of mat, each fiber strand will normally consist of 200-400 or more filaments per strand. In stamping of glass fiber reinforced thermoplastic, however, extreme shear and mechanical working is not encountered, and the fiber bundles are not filamentized or broken up into the individual filaments. This is in distinction to thermoplastic fabricating processes such as injection molding or extrusion, in which the glass fiber strands are broken up into the individual filaments and dispersed into the matrix. The breaking up and dispersion of these filaments is an important factor in obtaining good properties, since adequate wetting and contact of each filament by the matrix is thereby achieved. In stamping of long-glass fiber-reinforced thermoplastics, however, extreme shear and mechanical working is not encountered. Rather, compressive stresses and mild, somewhat limited, translational motion of the reinforced thermoplastic is encountered. This results in flow of the entire filament bundle or strand rather than breaking up of the strand into the individual filaments. In a tight collection of many filaments, it is clear that many filaments will not be wetted by the thermoplastic matrix, resulting in poor mechanical properties.